1. Field of the Invention
The present invention relates to a method and system for laying and burying pipe in a seabed using a plow apparatus that is advanced along the seabed to form a continuous trench. The plow apparatus of the present invention performs the burying of the pipe with minimal turbidity and other seabed disturbance making it attractive for environmentally sensitive applications. More particularly, the plow apparatus of the present invention includes a pipe guiding sled and a pipe trenching plow. The pipe guiding sled cradles and supports the pipe and guides the pipe downward for laying in the seabed. The pipe trenching plow cuts the seabed and using expandable spreading side sections widens and forms a trench. A trench box section retains the trench geometry and lays the pipe into the trench as the plow apparatus is continuously advanced in a forward direction.
2. Description of the Related Art
There are commonly known a wide variety of prior art systems for laying and burying pipes in seabeds. As discussed in this description, the term xe2x80x9cseabedxe2x80x9d refers to any water bottom where pipe may be laid including, for example, ocean bottoms, lake bottoms, river bottoms or canal bottoms. The types of pipe generally laid and buried by these systems are of varying diameters and materials and may include, for example, oil and gas transportation pipes, communications cabling, sewerage and water pipes and other utility transportation pipes.
Generally, these systems use either a plow or a jetting machine or some combination of both to cut the seabed trench for laying the pipe. These systems are generally towed along the pipeline path either by a surface vessel or by an underwater tractor machine and may be used to bury an existing pipeline or to lay a new pipeline.
Pipe laying systems that use a conventional plow to cut the seabed usually cut a V-shaped trench. In cutting such a trench, such plows stir up the seabed significantly and create a trench that is wider at the seabed then necessary. Such plow systems also are not as effective in seabeds with inconsistent soil textures cutting trenches of varying depth and width.
Pipe laying systems that use jetting machines typically are not effective in hard bottom applications and in soft bottom applications usually cause a very high level of turbidity and seabed disturbance. The trench formed by such pipe laying systems using jetting machines is also often wider than necessary and of an irregular geometry.
Thus, it is desirable for a pipe laying system to be effective to cut a trench of a geometry that closely fits the size of the pipeline to avoid unnecessary seabed disturbance. It is also desirable to avoid excessive turbidity in the cutting of the trench.
The pipe laying method and system of the present invention was designed for use in many subsea pipeline laying applications and is particularly intended for use in applications requiring minimal turbidity and seabed disturbance. The pipe laying method and system of the present invention causes minimal environmental damage to a seabed environment by reducing the disturbance to chemicals and other contaminants that may be settled in the seabed. Furthermore, in laying pipelines in environments near coral reefs or other biologically sensitive areas, the pipe laying method and system of the present invention minimizes the impacts of turbidity on such marine habitats.
The pipe laying system of the present invention includes a towable pipe guiding sled and a towable pipe trenching plow. The pipe guiding sled and the pipe trenching plow may be separate apparatus or they may be coupled together, the pipe guiding sled positioned forward of the pipe trenching plow. Both the pipe guiding sled and the pipe trenching plow engage the pipeline and may be towed along the pipeline path by a surface vessel or an underwater tractor. The pipeline may be an existing pipeline or may be a new pipeline that is in the process of being laid. The pipe guiding sled and the pipe trenching plow are equipped with bridles for harnessing by cable or chain to the surface vessel or underwater tractor machine for towing purposes. Alternatively, an underwater tractor machine may be mechanically coupled to either the pipe guiding sled or the pipe trenching plow for advancing along the seabed.
The pipe guiding sled includes a base and a pair of laterally spaced pontoons for engaging the seabed surface as the pipe guiding sled is advanced. The pipe guiding sled further includes a plurality of pipe guide assemblies mounted to the upper surface of the base for cradling the pipe and for guiding each pipe section at a consistent downward angle toward the trench. The degree of the angle of descent of the pipe will depend on the diameter and material properties of the pipe.
The pipe trenching plow follows the pipe guiding sled along the desired path of the pipeline and performs the actual cutting of the trench for laying the pipe. At the forward end of the pipe trenching plow is positioned a cutting apparatus that cuts a small pilot hole or groove in the seabed for advancing the pipe trenching plow. In one embodiment, such a cutting apparatus is a pair of vertically mounted counter-rotating cutters. Alternatively, such cutting apparatus could be a series of jet nozzles positioned along a tube member or plurality of tube members. Such cutting apparatus could also be a narrow blade or plowshare. Unlike prior art systems, the cutting apparatus is only used to cut the pilot hole in the seabed and not the entire pipe trench causing a significant reduction in turbidity. The choice of the cutting apparatus will typically depend on the characteristics of the seabed.
The pipe trenching plow further includes a pair of spreading side sections extending rearward from the cutting apparatus, the spreading side sections forming a plowing wedge. As the spreading side sections enter the pilot hole, the spreading side sections plow back the seabed to form a pipe trench. To further achieve the widening and formation of the trench, the spreading side sections are repeatedly expanded by a pair of hydraulically operated piston rod assemblies to form a rectangular pipeline trench. As the plow advances, the spreading side sections are continuously expanded and contracted to create a shoveling and packing motion on the seabed to widen and form the rectangular trench. Through this continuous motion of the spreading side sections, the pipe trench is cut and sized to the geometry required by the diameter of the pipe being laid. As compared to other prior art systems, the repetitive expanding and contracting motion of the spreading side sections as the plow system is advanced along the pipeline path effectively forms a pipe trench with reduced turbidity and disturbance to the seabed.
Following the spreading side sections of the pipe trenching plow is a trench box section, or as in the preferred embodiment, a plurality of trench box sections. In using a plurality of trench box sections, the leading trench box section is coupled to the spreading side sections and each subsequent trench box section is coupled to the preceding trench box section. Each trench box section has two side plates and a bottom plate, each side plate is braced by an outwardly extending pontoon. The trench box sections further pack and form the pipe trench and guide the pipe into the trench using pipe support rollers that cradle the pipe at the pipe""s angle of descent.
Finally, the pipe trenching plow includes a trench covering section that follows the trailing trench box section. The trench covering section includes a chute apparatus that allows the system user to cover the pipe by pouring a covering material, such as shells or sand, over the pipe after it is laid and rests in the pipe trench.
These and other features and advantages of the present invention will become apparent from the following detailed description, the accompanying drawings and the appended claims.